15 of the best Jennifer Lawrence quotes

An equally bemused Letterman, 66, said he "tried to Twitter" but had "nothing to say".

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At first glance they appear two outliers hopelessly missing the zeitgeist. After all, the most popular accounts in the world belong to the stars of film and music, led by Justin Bieber with nearly 45 million followers.

But a quick search reveals that Letterman and Lawrence are not alone. Many of the world's biggest stars are avoiding the Twittersphere altogether.

Among them is the best-paid actor of 2013, Robert Downey Junior, and the best-paid actress, Angelina Jolie.

Also absent is Madonna, the best-selling solo artist alive today, and names like Will Smith, Brad Pitt, Kirsten Stewart, Mila Kunis, Jon Stewart, and Robert Pattinson.

Sydney-based social media consultant Jeff Bullas says Twitter can be an extremely effective way of boosting a star's profile and building a fan community.

But he said some have such massive brand recognition that they see no need to tweet.

Others may be deterred by the same sorts of concerns that plague regular folk, such as privacy.

"Everyone's got a different degree of what they can tolerate in terms of being public. Some people are much more private than others," he said. "Social media can be very intrusive."

In addition, maintaining a Twitter account - especially one with millions of followers - can become an unwanted obligation. (Though this evidently doesn't concern Beyonce, who averages about one tweet every two months.)

Factor in the possibility of an account being hacked or hijacked, and it becomes easier to understand why many stars avoid the platform altogether.

But Bullas says all celebrities, especially those just starting out, should consider it.

Twitter and other social networking sites give stars "control" over their image - a means to bypass the traditional media gatekeepers of TV, magazines, and newspapers.

"No one is forever. You can lose a movie contract or a recording contract, and then you don't have that distribution any more."

"If you build your own network you've got control." But there are some, like Scarlett Johansson who will probably never come around.

She told the entertainment website Monsters and Critics that all social media "drives me crazy".